The present invention is directed generally to heat exchangers for power generation systems, and more particularly to heat exchangers which use heat transfer tubes as a heat transfer boundary, such as those of tube and shell construction, for transferring heat from a primary to a secondary fluid circulation system; wherein, upon reduction in the flow rate in the secondary system, selected heat transfer tubes can be plugged to increase the fluid flow rate through the remaining heat transfer tubes for reduced sedimentation.
Heat exchangers are utilized in power generation systems for removing heat generated within turbine generator sets. Typically, such heat exchangers transfer heat between a primary purified fluid system which conveys heat from the generator set to the heat exchanger, and a secondary system which circulates cooling water, often containing emulsified particles and contaminants, to convey heat from the heat exchanger to an external pond, lake or cooling tower. Within the heat exchanger fluid from the secondary system is caused to flow through an array of parallel-spaced heat transfer tubes, and fluid from the primary system is caused to flow around the tubes so as to establish thermal communication between the two systems
The heat transfer capacity of a heat exchanger in a power generation system must be sufficient to transfer a maximum heat load under the most unfavorable operating conditions, e.g., maximum supply temperature, minimum coolant flow, maximum fouling, etc. Consequently, heat exchangers for power systems are ordinarily over-designed to this condition, and have excess heat transfer capacity under more favorable conditions. In particular, a heat exchanger may have a greater number of heat transfer tubes than is required to achieve the necessary degree of heat exchange during normal system operating conditions. The excess heat transfer capacity of the extra tubes results in an undesirably low fluid flow rate through the tubes and a consequent increased tendency toward corrosion within the tubes.
This problem is particularly prevalent in northern climates where heat exchangers in power generation systems are exposed to wide variations in cooling water temperatures. As the inlet temperature of the cooling water drops the thermal driving force increases due to an increased fluid temperature difference, and as a result a lower flow rate is called for in the secondary system. If an undesirably low flow rate is established sedimentation and corrosion may occur in the heat transfer tube.
To eliminate excess heat transfer capacity and maintain a flow rate sufficient to avoid sedimentation in the heat transfer tubes the present invention contemplates plugging one or more of the heat transfer tubes, thereby forcing cooling water to pass through the remaining unplugged tubes at a higher velocity and reducing the tendency for sedimentation and corrosion. If at a later date increased cooling is required, the plugs can be removed and the associated heat transfer tubes returned to service. In essence, the isolated heat transfer tubes become installed spares.
For optimum performance, it is desirable that the tubes be plugged from both end-s and that the plugging be accomplished without modification to the heat exchanger or the heat transfer tubes. To this end, the present invention provides a plug assembly which includes compression-actuated sealing elements at each end of the tube which fluid-seal the open ends of the tube. In a preferred form, the compression-actuated sealing elements at each end include a cap member, which deforms over the open end of the tube, and a plug member, which deforms within the tube in cooperation with the cap member to tightly seal the tube. A spacing member in the form of an externally threaded rod extends the length of the heat transfer tube and through a central bore in the cap and plug members. This rod is fitted at each end with fastening means in the form of complementarily threaded machine nuts positioned to maintain the sealing elements at each end in compression.
In an alternative form of the invention where only one end of the heat transfer tube is accessible, the compression member is fitted with a sleeve at one end so that the cap and plug members at that end can be positioned at the inaccessible end of the heat transfer tube by sliding the members through the tube, after which the sleeve is utilized to force the plug member into compressive sealing engagement with the cap member at the inaccessible end by tightening a machine nut on the compression rod against the sleeve at the accessible end of the heat transfer tube.